1. Field of the Invention
The present invention relates to systems and methods for transmitting/receiving data, and in particular to a system and method for on-orbit reconfiguration of beams transmitted/received by satellite antennas.
2. Description of the Related Art
Commercial and military satellites often require the flexibility in terms of changing the coverage size and the beam location over the global field-of-view. It is also important to keep the feed(s) stationary for most applications either due to the high power required to carry multiple frequency channels on-board the satellite or to avoid long cables required to move the feed(s).
Many existing satellite designs have fixed beam coverages and therefore can not provide any flexibility in terms of coverage patterns on ground and also can not be adapted to changing service requirements once the satellite has been launched.
Future applications for both commercial and military satellites may require the beam shape as well, as the beam location to be reconfigured over the global coverage based on changes in traffic demand, changes in the coverage scenario and/or the need for a service back-up for an on-orbit or launch failure. This flexibility is critical to many satellite operators in order for them to provide uninterrupted service to their customers.
Existing methods of beam reconfiguration involve either moving the feed of a reflector antennas or use of phase array antennas. These are risky due to the high power going through the feed, long and glossy cabling requirement, or very expensive hardware with increased power consumption on satellite.
In the paper “Variable Beamwidth Dual-Reflector Antenna’, IEEE Conference on Antennas & Propagation (ICAP)”, Publication # 407, pp.92-96, April 1998, which is hereby incorporated by reference herein, authors J. U. I. Syed and A.D. Olver describe a method of changing the beam size by moving the feed of a reflector antenna. They employ a symmetrical Cassegrain reflector antenna with main and sub-reflectors which inherently has high sidelobes and low beam efficiency due to blockage effects caused by the feed and the sub-reflector. This method has limited beam shape reconfiguration due to the fact that the main beam splits or bifurcates for beam aspect ratios greater than 1:2.5 and therefore resulting in poor gain performance.
In another paper, “A Novel Semi-Active Multibeam Antenna Concept”, IEEE Antennas & Propagation Symposium Digest, pp. 1884-1887,July 1990, authors A Roederer and M. Sabbadini describe a semi-active multibeam antenna concept for mobile satellites. The beams are reconfigured using a Butler matrix and a semi-active beamformer whereby a limited number of feed elements (typically three or seven) are used for each beam and the beam reconfiguration is achieved by varying the phases through the active BFN. This scheme provides limited reconfigurability over a narrow bandwidth and employs complicated and expensive hardware.
U.S. Pat. No. 6,198,455, entitled “Variable Beamwidth Antenna Systems” and issued to Luh on Mar. 6, 2001, which is hereby incorporated by reference herein, describes an offset dual-reflector antenna in the Gregorian configuration. This requires feed movement and also reflector movement (main or sub-reflector) and also has limited range of beam size reconfiguration (beam size aspect ratio of less than 1:2) due to the use of single feed and has disadvantages associated with feed movement.
U.S. Pat. No.5,859,619, entitled “Small Volume Dual Offset Reflector Antenna”, and issued to T. Wu, B. Yee and G. H. Sinkins on Jan. 12, 1999, which is hereby incorporated by reference herein, describes a compact dual-offset Cassegrain antenna system that requires the position of the feed, position of the sub-reflector and the feed axial direction that need to be changed in order to arrive at a compact antenna configuration. This is mainly intended for fixed beam applications and does not provide the beam size flexibility.
What is needed is an antenna system that provides for control of the beam size as well as the beam direction, and is compatible with a high-power and stationary feed array requirements. What is also needed is a system that extends the range that the beam size can be reconfigured and provides high beam efficiency values over the beam zooming range while minimising scan loss. The present invention satisfies that need.